Conflicting phosphate readings

I'm getting conflicting phosphate readings from the test kits I am using. I consistently got a phosphate reading of 0 from the API test kit I was using since January of this year. Then I purchased the Hanna Instruments "Checker" for phosphates and I'm getting a reading of 0.06. So, I pulled out the API test kit to double check and I got a 0 reading. I used RO/DI water to fill my 90 gal. in December and use only RO/DI water to top off. Which test kit results would the forum go by? If the checker results are correct, where are the phosphates coming from? And are the checker reading of 0.06 considered high?

 

Phosphates are in basically all foods. Not considered dangerous levels, unless you keep SPS.

 

I would tend to trust the checker more than test kits personally but that's just because reading color charts is difficult and I think can vary quite a bit from person to person. Are you sure you're running the checker test properly, doing the zeroing out procedure correctly, with clean glass etc?

How old is your API test kit? I would put a much higher margin of error on the API kit. Maybe try this, put a small amount of fish food in a cup of your tank water and let it soak for a while, maybe 20 minutes, and then test that water and see if the API kits registers something, maybe check that with the checker too. If nothing registers, keep letting the food soak until you do get something to register. Your kit may just be testing zero for everything, meaning it's just expired or contaminated/ruined somehow.

 

So on Hanna's website they say this about the accuracy.

The HI 713 Checker®HC portable handheld colorimeter features a resolution of 0.01 ppm (250 points) and 4% ±0.04 accuracy of reading

Does this mean the reading of .06 could be .02 or .10? Just curious? I've been considering buying one. Instead I've been using the Mereck Phosphate Test Kit which seem to be as accurate as the photo meters.

 

The other thing is that any algae will consume phosphates so testing that won't really give you a good reading.

 

According to Hanna it is +/- 4% PLUS +/-.04. So it's in reality slightly less accurate than +/-.04. There are some things you can do to help make sure you're getting accurate readings though. Keep in mind this is the same accuracy as the expensive phosphate tester they've had out for a long time and they have been tested by some people to produce similar results well within the accuracy range.

I like 3reef and I hate sending people to other sites but there is too much to summarize easily and I think it's somewhat important if you're using these checkers. I would recommend reading this thread on RC, there is a long discussion about how best to use the checker and all of the details on how to get consistent results and avoid errors etc. Hanna weighs in at some point as well in the thread. Make sure you read until the end (you could probably skip the first couple pages) since some assumptions made in the beginning are not held true to the end.
Hanna HI 713 Phosphate colorimeter - Reef Central Online Community

 

This is true, my understanding is (and I'm not a chemist, just have read a little about this) a 0.0 reading (even if it was true, and not +/- .04) will not tell you for sure that there are no phosphates in the tank. But it will tell you that there isn't any "free", or floating around at least at the time you check it, which is a good thing. You don't want it measurable, or only very slightly measureable, since corals do need SOME phosphate to live, all life does.

Basically, if you get a 0.0 reading it's still a good thing it just may mean that your refugium, or that GHA patch that you can't get rid of is sucking it all up.
You can't avoid adding phosphate to the tank, like jon said above, it's in ALL food, you just have to make sure you have enough systems in place to remove it before it has a chance to affect your coral.

If you are getting a positive reading it means that you do not have enough nutrient removal systems in place and if not remedied it will probably start to impact your coral if it gets out of hand (of course some corals are more sensitive to excessive phosphate than others), or it could lead to algae problems.

Anyways, that's my understanding of it, please feel free to correct me, anyone. ;D

 

That is exactly how I understand it too.

 

Thanks everyone. After reading the thread on RC, I know I'm not the only one with questions about this issue. Karma to everyone who responded.